Ophthalmological laser-therapy instruments are used for operations in the front and middle regions of the eye. The laser radiation is conducted through the lens of the ophthalmological instrument and is focused on the eye of the patient. The source of the laser-therapy beam (sometimes identified as the instrument's "active" radiation) is generally a Nd-YAG laser which emits in the invisible region of the spectrum. Therefore, in order to check the focal plane of the active (i.e., therapy) radiation, the instrument has an observation ray path for viewing radiation in the visible region of the spectrum, and visible "marking" radiation is superimposed on the invisible therapy radiation. Such marking radiation is generally provided by a He-Ne laser which is arranged so that its focal plane coincides with the focal plane of the active radiation. By adjusting the focal plane of the marking beam, it is possible to determine and control the focal plane of the invisible active radiation being used for therapeutic purposes.
An instrument of this type is described in detail in U.S. Pat. No. 4,582,405, which also discloses a mechanism for determining the focal plane of the invisible active beam. This prior art mechanism divides the marking laser beam into two portions which are sequentially and alternately blocked. When the beam path is intercepted by a plane which is not identical with the focal plane of the instrument's radiation, the user has the impression that the marking radiation is blinking. This prior art focusing arrangement requires beam-widening means to be located at two places along the path of the marking beam, and it also requires occulting means for alternately blocking the respective portions of the marking beam.